Because geologic units occurring at the same time but from different parts of the world can often look different and contain different fossils, there are many examples where the same period of time was historically given different names in different locales.

www.wikipedia.org /wiki/Geologic_timescale (1711 words)

Encyclopedia: Paleozoic(Site not responding. Last check: 2007-10-21)

The Cambrian is a major division of the geologictimescale that begins about 542 million years before the present (BP) at the end of the Proterozoiceon and ended about 490 million years BP with the beginning of the Ordovician period.

The Devonian is a major division of the geologictimescale that extends from the end of the Silurian period (360 million years ago (mya)) to the beginning of the Mississippian subperiod of the Carboniferous (408.

Geologically, the Paleozoic starts shortly after the breakup of a supercontinent called Rodinia and at the end of a global ice age.

When used in formal writing, specific geological and historical periods and events are capitalized.

This is largely due to uncertainties in radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where we would most like to have them.

encyclopedia.worldsearch.com /geologic_time.htm (456 words)

Geologic Timescale(Site not responding. Last check: 2007-10-21)

Geologic Time USGS on-line publication about the relative and radiometric geologic time scales, age of the earth, index fossils, and related topics.

Bedrock Geologic Map of Rhode Island A reproduction of the state geological survey's map, with a summary description of the state's geologichistory.

Carbon-Dioxide Sequestration in Geologic Media Texas Bureau of Economic Geology describes its research initiatives on the potential to sequester carbon in saline geologic formations and abandoned oil/gas reservoirs.

The numerically calibrated geologic time scale has been continuously refined since approximately the 1930s (e.g., Holmes, 1937), although the amount of change with each revision has become smaller over the decades (see fig.

The overall duration and relative length of these large geologic intervals is unlikely to change much, but the precise numbers may "wiggle" a bit as a result of new data.

Geologic time is finely subdivided through most of the Phanerozoic (see Harland et al.

These subdivisions were devised on the basis of fossil and geologic evidence of global change.

By moving your mouse over each era you can see the divisions into the different periods and clicking on the timescale will bring you to a text on some of the major events that took place.

www.mnh.si.edu /anthro/humanorigins/faq/gt/gt_scale.html (250 words)

Palaeos Timescale: The Geological Time-Scale(Site not responding. Last check: 2007-10-21)

Scientists divide the Earth into a number of periods - the "Geological time-scale", according to the rock types and sort of fossils found in each one.

The Geological time-scale can also be used to define the major stages in the history of life on Earth.

Most recently, in the blink of an eye geologically speaking, this era saw the rise of Man (Homo erectus, Neanderthal and Cro Magnon) and use of stone tools and fire, the extinction of Megafauna, and civilization and human activities that have transformed the globe, but at a cost of great environmental destruction.

www.palaeos.com /Timescale (960 words)

Lunar geologic timescale(Site not responding. Last check: 2007-10-21)

The lunar geologictimescale divides the history of Earth 's Moon into six generally recognized geologic period s:

Pre-Nectarian Epoch: -4550 MY to -3920 MY The values for these dates remain in some dispute, as the divisions are based on the dates when various significant lunar geological events occurred and it is difficult to pinpoint them exactly.

The geologictimescale is used by (A specialist in geology) geologists and other scientists to describe the timing and relationships between events that have occured during the (A record or narrative description of past events) history of the (The 3rd planet from the sun; the planet on which we live) Earth.

The (The 3rd planet from the sun; the planet on which we live) Earth is thought by geologists to be about 4570 million years old.

This is largely due to uncertainties in (Click link for more info and facts about radiometric dating) radiometric dating and the problem that deposits suitable for radiometric dating seldom occur exactly at the places in the geologic column where they would be most useful.

The GeologicTimescale was initially built from many observations of rock sequences from many parts of the world; no single locality contains the entire geologic time sequence, and correlation of geologic units across different localities allowed geologists to construct a composite timescale.

The guiding principle followed by many geologists in the 1800's was that current rates of geologic processes were more likely than not to be similar to those in the past - this principle is called uniformitarianism, and was proposed by geologist James Hutton.

Geologists immediately started determining the ages of volcanic rock layers that were interbedded with the fossil-containing sedimentary rocks that were used to determine the first geologictimescale, adding absolute ages to the geologic eras and periods.

Almost all of the dates on my 1994 Pan Terra geologic chart are wrong by a few million years.

In 1987, the period was estimated to have ended 131 million years ago, based on the amount of potassium that had been converted into argon in a mineral called glauconite.

But it was later discovered that argon seeps out of glauconite, making the mineral seem younger than it actually is. The new timescale used potassium-argon dating of basalt to put the end of the Jurassic at 145.5 million years ago.

As the basis of the BGS Geological Timechart they have chosen the timescale of Gradstein and Ogg (1996) for the Phanerozoic and that of Cowie and Bassett (1989) for the Proterozoic (Precambrian).

ICS (within the International Union of Geological Sciences) is the only organisation concerned with stratigraphy on a global scale.

This multi-year project summarized the history and status of boundary definitions of all geologic stages, compiled integrated stratigraphy (biologic, chemical, sea-level, magnetic, etc.) for each period, and assembled a numerical age scale from an array of astronomical tuning and radiometric ages.

efore modern radiometric dating techniques made it is possible to determine rock ages empirically, the geologic record was divided into relative time units based on correlation of the rock formations, mostly by the index fossils they contained.

This is accomplished by measuring the amounts of certain radioactive isotopes contained in the rocks (usually limited to igneous rocks), in relation to the amount of corresponding decay isotopes (daughter products) that are present.